This invention relates generally to fiber optic sensor systems and particularly to techniques for processing signals output from fiber optic interferometric sensors to measure changes in a physical parameter.
A time division multiplexed array sends a pulse of light from a laser source down a fiber optic transmission line toward a series of separate fiber optic interferometer sensors. With suitable delays in the fiber optic network, the single input light pulse is divided up among the interferometer sensing elements. Each element has an output light pulse with the appropriate acoustic information encoded on it. The output light pulses are coupled sequentially with no overlap onto a return fiber optic transmission line that travels to the photodiode receiver and associated signal processing electronics. Thus, one input light pulse is converted into a train of output light pulses equal to the number of sensors.
The time duration of the output pulse train governs the repetition rate of the input pulse. After one output pulse train is received, it is closely followed by another pulse train derived from a second input pulse. The duty cycle of the input pulse train is low. It can only approach one over the number of sensors interrogated without overlapping of adjacent output pulses.
There are two methods of input light pulse generation, internal and external. The internal method is the on and off switching of the laser source, and the external method is the on and off gating of a constant output laser source with an external optical switch. In either case, ideal switching implies that the off state is truly off with an infinite on-off extinction ratio allowing for no unwanted background leakage light.
For a non-ideal switch, leakage light traveling through the fiber optic acoustic array is superimposed on each output light pulse as it is electronically gated and detected. In a fiber optic system with a large number of sensors, the noise produced by even a very small amount of background leakage light can be many times the noise produced by the same output light pulse with no light leakage. In particular, intensity fluctuations of the leakage light from numerous parasitic interferometer returns are produced by laser phase noise. For N sensors the number of parasitic interferometer returns is equal to N*(2N-1). Each of these returns has a noise power associated with it. This component of optical noise, which is primarily 1/f in character with most of its frequency content below 100 KHz, can severely degrade system performance even with a 50 dB optical switch.
The parasitic interferometer returns for N sensors correspond to a variety of pathways. Length mismatches are as short as the length mismatch for each of the sensors and as long as the difference between the longest round-trip path through the most distant sensor and the shortest round-trip path through the closest sensor. The length mismatch for each sensor which will be called the primary length mismatch is typically one meter whereas the longest mismatch might be one kilometer or more. The second shortest mismatch corresponding to round trip distances between adjacent sensors is typically twenty meters or more.